1. Field of the Invention
The present invention relates to open-pore microporous, macroporous and mesoporous materials, such as aerogels, and particularly to processes and methods for making high strength aerogels which do not require supercritical solvent extraction or solvent exchange drying techniques, and to such high strength, low density aerogels and/or xerogels. More specifically, the present invention relates to low-density, high strength, monolithic aerogels and/or xerogels made from polyhydroxy benzenes and aldehydes in defined ratios
2. Description of Related Art
Aerogels are a special class of open-cell foams derived from highly crosslinked inorganic or organic gels. These materials have fine or ultrafine cell/pore sizes, continuous porosity, high surface area, and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 10 nanometers (nm). This microstructure is responsible for the unusual optical, acoustical, thermal, and mechanical properties of aerogels. By definition, these materials are prepared through the sol-gel process and can be either granular or monolithic.
Aerogels are characterized by the presence of nanometer size pores and particles, wherein size may be dependant on the density (porosity) and the chemistry of formation. Organic aerogel particles may range in size from about two nanometers to about twenty microns. Aerogels may be made with dimensions of the pores and particles less than that of the wavelengths of visible light, resulting in transparency and other exceptional properties. By increasing the size of the particles and/or pores, opaque aerogels may be produced.
Aerogels are also characterized by the nature of their porosity (e.g., pore size and pore distribution. Typically, they possess micro-pores, meso-pores, and macro-pores. Micro-pores generally include pores with dimensions less than about 2 nm; these are often pores within or between individual particles. Meso-pores are generally pores with dimeinsions about 2 nm to about 50 nm; these are often associated with the spacing between particles or chains of particles. In aerogels, macro-pores are generally pores with dimensions about 50 nm to about 100 nm. The high surface areas of aerogels (e.g., 300 to 1000 m2/g) is attributed to the porous nanostructure.
Organic aerogels result from the reactions of certain organic compounds, for example (1) resorcinol with formaldehyde (known as RF aerogel); (2) melamine with formaldehyde (known as MF aerogel); and (3) phenolic-furfural with propanol. Such aerogels can be prepared in monolithic form and have been employed in numerous applications, including dielectric materials, such as capacitors.
In the case of the preparation of organic RF aerogels, a phenolic substance such as resorcinol is combined with an aldehyde, such as formaldehyde, in the presence of at least one catalyst. The gels so formed can be exchanged into an organic solvent and/or supercritically dried with C02. Examples of preparations of organic and carbon aerogels can be found in U.S. Pat. Nos. 4,873,218; 4,997,804; 5,086,085; and 5,476,878, issued to Pekala, the disclosures of which are incorporated by reference herein in their entireties.
In the manufacture aerogels, the porous gel material is dried by removing the liquid from a two-phase liquid-solid network. Aerogels of the prior art are dried using special techniques to preserve the tenuous solid network. The network of one type of microporous material, an organic aerogel, is typically produced in a two-step process. The first step comprises the “sol-gel” chemical reaction that produces the aerogel structure in solution. In this step, sols are formed, and the sols link until a connected solid structure (a gel) forms, which is surrounded by a liquid by-product of the same reactions.
In the second step, after gelation is complete, and often after additional time for curing, the gel is dried by removing the liquid from the pores. Ideally, this is accomplished in such a way as to only minimally change the porosity of the wet gel. Depending on the concentration of reactants and/or catalyst in the mixture, the gelling step can occur in minutes or hours, but typically takes days, and can require weeks for dilute mixtures.
The small size of the pores results ill useful physical properties, but also complicates the drying of the wet gels to aerogels. The pore sizes in the material are so small that the flow of liquid to the surface from within the gel is limited. In evaporative drying, the surface tension of the liquid in the small pores creates extremely high forces as the material dries, which tend to collapse the weak solid structure of the gel. The gels are typically not strong enough to resist this shrinkage during evaporation. The forces become significant when the pores become less than one micron in diameter. For porous solids like aerogels, whose average pore size is much less than one micron, evaporative drying, if applicable at all, must be done extremely slowly to minimize cracking and shrinking. These long drying times can also limit large scale production.
Techniques for improving the process of drying gels to aerogels involve one of two basic ideas. One idea is to modify the surface energy of the gel chemically. The surface tension forces are reduced below the basic strength of the gel, so the liquid is removed with minimal stress to the gel. These methods may require hours or even tens of hours to complete the entire process of making and drying the gels.
Solvent exchange methods comprise substituting the original solvent used in the gel-sol process is substituted for a solvent of lower surface tension. The process typically requires several solvent substitution steps to gradually increment down the solvent surface tension to a point wherein the gel can be air dried. Solvent exchange drying methods are deficient in that they require multiple steps and multiple solvents, and are generally not effective in making monolithic aerogels with very low densities.
The other idea is to change the temperature (or the pressure) of the gel so that the pore liquid is transformed to another state of matter (i.e., solid, gas, or supercritical fluid). The new state has reduced interfacial surface tension with the gel and can be removed from the gel without excessive shrinkage, either by evacuating or purging with a gas.
Supercritical fluid extraction (or drying) exerts no surface tension across the cells and pores of the aerogel structure, preserving the integrity thereof. Monolithic aerogels made in accordance with the teachings of the prior art, in particular, necessitate supercritical drying of cross-linked organic or inorganic gels because of their extremely line pore size. Where monolithic aerogeis are desired, it is important to preserve the porosity and particle distribution in order to preserve the monolithic structure. Thus the drying step is important in making monolithic aerogels.
In supercritical drying, the gel is placed in an autoclave where the temperature and pressure are increased above the supercritical point of the fluid in the pores. This technique has a drawback. The gel may crack during heating because the liquid solvent within the gel expands faster than it can flow through the very fine pores, thus causing tension and internal stresses in the gel. To avoid cracking, the stresses cannot exceed the basic strength of the gel (i.e., its modulus of rupture). The rate of heating the gel must be slowed so that the rate of expansion of the liquid solvent does not stress the gel beyond its modulus of rupture.
A related conventional method comprises freeze-drying, where the liquid is cooled to a solid and sublimed. As with supercritical drying methods, freeze-drying methods are time-consuming, energy-intensive, and require additional materials processing. They are inherently batch processes and not amenable to rapid processing for mass production. Drying through supercritical solvent extraction is further energy intensive and requires specialized equipment.
Such drying techniques of the art are disclosed in U.S. Pat. Nos. 4,873,218 and 4,997,804, both the Pekela, which relate to low density resorcinol-formaldhyde aerogels, dried by supercritical solvent extraction and/or solvent exchange.
U.S. Pat. No. 6,005,012 to Hrubesh et al. describes a method for preparing monolithic, transparent aerogels having hydrophobic properties. The aerogels of Hrubesh et al. rely on solvent exchange and/or supercritical extraction to dry.
U.S. Pat. No. 5,686,031 to Coronado et al. is exemplary in disclosing a method for making microporous and mesoporous materials, including aerogels wherein a gel precursor liquid is heated and allowed to generate internal pressure within a confining vessel, and a supercritical extraction is employed to remove the internal gel liquid from the gellation process.
The prior art, as exemplified by the foregoing references, is consistent in teaching organic aerogeis made using a stoichiometric or near stoichiometric R:F ratio of about 1:2 or about 1:1.5, or greater ratios of resorcinol to formaldehyde. The art further teaches that only the final total solution volume (to dictate an “R” or % solids value), and the catalyst concentration (R/C ratio) can be varied to vary particle and/or pore size of the resulting gel.
In addition to the R:F ratio, the molar concentration of resorcinol to catalyst (R/C ratio) is considered important to controlling properties of the gel thereby produced. In the art, it is known that the size and number of resorcinol-formaldehyde clusters generated during the polymerization may be controlled by the R/C ratio. R/C values of 50-300 provide an acceptable range in which transparent gels can be synthesized. Outside of this range, opaque gels or precipitates are usually obtained.
The art teaches that, for organic aerogels, the concentration of resorcinol to catalyst (R/C) ratio is the dominant factor which affects the density, surface area, and mechanical properties of RF aerogels. These aerogels are composed of interconnected colloidal-like particles derived from the clusters formed in solution. Under high catalyst conditions (i.e., R/C =50), the particles have diameters of 3-5 nm and are joined together with large necks, giving the aerogel a fibrous appearance. Under low catalyst conditions (i.e., R/C =200), the particles have diameters of 11-14 nm and are connected in a ‘string of pearls’ fashion.
U.S. Pat. No. 5,945,084 to Droege discloses methods by which density of an organic aerogel, is modified. By using ratio of resorcinol:catalyst of about 2000:1, Droege attempted to decrease the size of nucleation sites to result in larger particles and pore sizes, in order to be able to air dry the resultant aerogel. Droege teaches, as do others in the art, that a stoichiometric ratio of resorcinol:formaldehyde (R:F) preferably a slight excess of formaldehyde, be used.
The art has thus focused on modifying the density of the aerogel, or modifying the RIC ratio, in order to produce low-density monolithic areogels.
The art is deficient in teaching, disclosing or suggesting any means of drying other than by supercritical fluid extraction as a means of drying an aerogel to obtain a sufficiently strong, low density monolithic product.
A need exists, therefore, for a method by which gels may be made which can be air dried into continuous porosity, low density, high strength, organic foams or aerogels.
A need exists, therefore, for a method by which gels may be made which can be air dried into monolithic, low density, high strength, organic foams or aerogels.
A need therefore also exists to produce monolithic, or continuous porosity, low density organic foams or aerogels which do not require energy or equipment intensive drying methods, such as supercritical solvent extraction, but instead can be air dried.
Therefore, in view of the foregoing, there is a need to solve one or more of these disadvantages of the prior art products and methods.